The passive intermodulation (PIM) interferences that are observed at wireless communication sites are usually generated from two or more high power carriers which are mixed together in a non-linear passive component, such as rusty metal, located in the vicinity of the transmit antennas of the radios at the site. Those intermodulation products are especially harmful when they are present in one of the uplink receive channels of the radios since the undesired intermodulation products deteriorate the signal to interference plus noise ratio (SINR) as well as the link budget.
FIG. 1 is a block diagram of a conventional wireless communication system 10 having radios 12a and 12b, referred to herein collectively as radios 12. Each radio 12 may serve, via antennas 13a and 13b, multiple wireless devices 14a and 14b, referred to herein collectively as wireless devices 14, which may include but are not limited to mobile phones, laptop computers, tablet computers, other wireless devices, etc. Each radio 12 may be in the same base station 15, such as an eNodeB in a long term evolution (LTE) wireless communication system or may be in different base stations. Each base station 15 or radio 12 is in communication with a network 16 which may include the Internet and the Public Switched Telephone Network (PSTN). In a typical wireless communication system there are many base stations and many wireless devices 14. Each base station 15 may have multiple antennas 13 and multiple transmitters and receivers. Further, more than one base station 15 can be located at the same location, referred to herein as a cellular site.
The wireless communication system 10 may also include passive intermodulation (PIM) interferences 18. PIM interferences have always existed in the vicinity of high power transmitters. However, they are becoming a problem for cellular communications in crowded spectrums where the intermodulation products can be present in the selected uplink channel, thus desensitizing the collocated receivers. PIM is a form of intermodulation distortion that occurs in passive components such as antennas, cables, connectors or duplexers with two or more high power input signals. PIM can also be generated by rust, corrosion, loose connections, dirt, roofing materials and oxidation. When PIM is present, two or more signals can mix and generate interference signals. Self-generated PIM is caused by a base station's own signals interfering with one another. PIM may also arise when a base station's signals interfere with signals from external sources. Further, PIM may result due to interference of external signals only.
PIM interference may be generated by the interaction of strong radio frequency (RF) signals of one or more radios and PIM sources. The resulting PIM interferences will exist at a set of predictable frequencies that are dependent on the frequencies and bandwidths of the strong RF signals. The PIM signals may be in a conducting path of the RF signals, such as within the radio, connectors, cables and antenna, or may be external to the conducted path, i.e., outside the antenna and radio.
In some cases, strong RF signals on the downlink carriers supported by the radio that mix at the PIM source are known to an algorithm or module that is capable of performing PIM detection and/or cancellation. These RF signals can be obtained by, for example, sniffing the cable that connects to the RF front end of the radio to a digital baseband unit. The RF signals can also be obtained by tapping the signals somewhere along the radio's transmission path, such as by use of a directional coupler, and then sampling the obtained signal by a receiver. In some cases, the downlink carriers are not known and may be determined by over the air sampling. The downlink carrier frequencies may be used to determine frequencies at which PIM occurs.
However, known methods do not detect PIM products generated by non-linear mixing of two separate radio bands. Many radios have the ability to detect and measure if there is interference in a received signal, but cannot determine the type of interference or whether the interference is being generated by PIM products at the base station. If PIM is suspected, a visit to the base station site may be needed to perform further measurements to determine if PIM is present.
There are also standalone PIM test tools that generate two closely spaced continuous wave test tones within a radio band and detect the returned close-in third order modulation (IM-3) product. These instruments can be used at the base station site, but they do not measure mixing of products of transmit signals of two different radios, and are unlikely to detect PIM resulting from the mixing of RF signals from the different radios.